Introduction to Unix PDF

Introduction to Unix A brief history of UNIX Why use UNIX? A model of the UNIX environment The UNIX file system (directories, commands) File attributes (permissions)... and much more. University of Victoria SENG265: Software Development Methods Department of Computer S...

Introduction to Unix A brief history of UNIX Why use UNIX? A model of the UNIX environment The UNIX file system (directories, commands) File attributes (permissions)... and much more. University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 1  Embrace the mood of voluntary disorientation for the next few weeks. University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 2 A brief history of UNIX UNIX is a: – multi-user, multi-tasking operating system (or “OS”) – architecture-independent operating system (“portable”) the “UNIX” trademark: – owned by AT&T – passed to the “Unix System Laboratories” (USL) – passed to Novell – passed to X/Open Company, Ltd. (1993) – X/Open + Open Software Foundation (OSF) ! The Open Group – The Open Group (1996), http://www.opengroup.org/ So every manufacturer calls it something else! University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 3 A brief history of UNIX (2) AT&T / Bells Labs (was Lucent Technologies, now Avaya & Alcatel-Lucent) – Unix created by two researchers for their own personal use (Thomson & Ritchie, 1970) – academic/research operating system – Initially, pros: flexibility, extensibility, file sharing – Initially, cons: security, robustness, performance – easy to use (in comparison with contemporaneous OSes) – the first portable OS where "portable" == “recompilable and executable on another instruction set architecture” University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 4 A brief history of UNIX (3) Berkeley Standard Distribution (BSD) – freeware! (cheap for universities; only paid for distribution cost) – first UNIX to include standard network support – enhancements to interprocess communication (IPC), job control, security many flavours of UNIX in use today: – FreeBSD, NetBSD, XENIX, Solaris, SunOS, HP-UX, Linux, A/UX, AIX, macOS continues to evolve – e.g., Single UNIX Specification (derived from POSIX standard) Free Software Foundation and GNU Unix Ubuntu – arguably the most popular Linux distribution CentOS 7 – used in ELW B238 University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 5 Android First introduced in 2008 – Android 1.0 – Running on device codenamed “Dream” (T-Mobile G1) Major versions have been released ever since – Current active release (October 12) is Android 12 (sadly: desserts no longer used for the names of releases) Other related variants have been released – Android TV – Wear OS (used to be Android Wear) – Android Auto University of Victoria SENG265: Software Development Methods Department of Computer Science Logos from: https://bit.ly/2vcPc7t, Introduction to UNIX: Slide 8 https://arris.ly/2LZKAst, https://bit.ly/2vhfJAf, https://bit.ly/2LVHRQG Aside 2: AOSP Android Open Source Project – https://source.android.com Code for a complete/full software stack – From web browser, media players, etc.... –... all the way down to USB, wifi, power management, etc. Dependent upon the Linux kernel – And much else besides (including Java) Note: The full AOSP distribution is much, much larger than the Android SDK – Takes around one hour to download – Just the source code is 100 gigabytes... –... and to make one build you’ll require another 150 gigabytes. Android uses Linux/Unix access controls and process isolation in very interesting & novel ways. University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 9 Why use UNIX? multiuser multitasking remote processing stable (some might even say "safe") highly modular some versions are free (open source ! freedom to modify) large user community, extensive experience “tools are mature” University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 10 UNIX model of user interaction University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 11 UNIX model University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 12 shell responsible for communication between the user and kernel “shell” refers to its position in some diagrams of UNIX structure – These diagrams use concentric rings to show layering reads and interprets user commands at the console (or from within a “shell script”) implements job control many shells have been developed: – sh, csh, ksh, tcsh, zsh, bash … – in this course we use the bash shell – bash extends sh, the Bourne shell University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 13 kernel the kernel is the protected core of the OS the kernel is itself a large and complex program clear demarcation between the “kernel” and a “user” – to access a computer’s hardware (via the OS), user's commands must go through kernel – that is, “user” must request the kernel to perform work on behalf of “user” – user/OS interaction mediated by a command shell (e.g., bash), or the system library (compiled application) main responsibilities – memory allocation – file system – loads and executes programs (assumes a process model) – communication with devices (input, output) – bootstraps the system – …University of Victoria Department of Computer Science SENG265: Software Development Methods Introduction to UNIX: Slide 14 UNIX filesystem “file”, “filesystem”: are key abstractions of the UNIX computing model practically anything can be abstracted as a file (devices, programs, data, memory, IPC, etc.) mainly responsible for mapping blocks of data within physical storage devices (hard drive, flash memory) onto logical blocks that users can manipulate – maps filenames to block numbers – handles block allocations; chains units together – provides methods to access data facilitates the “multiuser” view of the OS University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 15 UNIX filesystem arranged as a hierarchy (tree-like structure) – organized as a collection of directories; think of a directory as a folder – forward slash "/" is used to separate directory and file components (cf., Windows uses "\") the root of the filesystem is represented by the root-directory, which we denote by a single "/" University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 16 Part of a (hypothetical) UNIX filesystem tree University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 17 Some properties of directories directories are actually “ordinary” files information contained in a directory file simply has a special format every directory contains two special directory entries – ".." refers to parent directory in hierarchy – "." refers to the current directory (itself) '~' is used to denote a home directory – % cd /home/user ≈ cd ~user % cd ≈ cd ~ University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 18 Directory commands (ignore %) Example: % cd /home listing directories % ls zastre keyboardcat % ls keyboardcat hi-rez.mp4 tinder-stuff.txt relative pathnames % cd /home % open keyboardcat/hi-rez.mp4 % open./keyboardcat/hi-rez.mp4 % open./keyboardcat/../keyboardcat/hi-rez.mp4 University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 19 “working” vs. “home” directory “Working” directory is the directory the shell determines you are “in” at any point in time. – Eliminates the need to continuously specify full pathnames for files and directories – “Relative pathnames” are locations worked out in relation to (relative to) to the working directory. “Home” directory is (usually) configured to be your working directory upon logging into the system – Sometimes called the “login” directory – /home/zastre & /home/seng265 are typical home directories University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 20 Directory commands (2) traversing directories % cd /usr % ls ucb bin lib display the current working directory % pwd /usr creating a symbolic reference to a file (i.e., like an alias) % cd ~zastre % # ln –s % ln –s a1 sample_solution % ls a1 sample_solution University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 21  working directories # “cam” is the logged-in user 5 # # Each of the following commands # starts in Cam’s current directory 4 0 # /home/cam (i.e., every item # below assumes we reference # from at red-arrow 0). % cd book #1 % vi book/wonderland #2 1 7 % vi ~/book/wonderland #3 6, % cd /usr/lib #4 % cd.. #5 % cd../gryphon #6 % cd ~gryphon #7 3 2, % cd alice # ?? University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 22 File attributes every plain file and directory has a set of attributes, including: user name (owner of file) group name (for sharing) file size (bytes) creation time, modification time file type (file, directory, device, link) permissions % ls –l unix.tex test -rwxr-xr-x 1 joe users 200 Dec 29 14:39 test -rw-r--r-- 1 dmg users 21009 Dec 29 14:39 unix.tex University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 23 Who has permission? permissions can be set for – user ("u") [-rwx------]: the file owner – group ("g") [----rwx---]: group for sharing – other ("o") [-------rwx]: any other – all ("a"): user + group + other user: the owner of the file or directory; owner has full control over permissions group: a group of users can be given shared access to a file other: any user who is not the owner and does not belong to the group sharing a file University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 24 What kind of permissions? files: – read (r) : allows file to be read – write (w): allows file to be modified (edit,delete) – execute (x): tells UNIX the file is executable – dash (-) : owner/group/other have no permissions directories: – read (r): allows directory contents to be read (listed) – write (w): allows directory contents to be modified (create, delete) – execute (x): allows users to navigate into that directory (e.g, with the cd command) – dash (-) : owner/group/other have no permissions University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 25 chmod: set file permissions +: add info there are several ways to use "chmod" - : remove info – use letter symbols to represent "who" and "what" % chmod o+rx ~/.www/ppt # other can read and cd "ppt" % chmod u+x run.pl # script "run.pl" executable % chmod go-rwx ~/private # removing access group & other % chmod u=rwx,g=rx,o=x foobar.txt # all permissions – can also use "octal" (base 8) notation, representing each three-bit field with an octal digit; r Î {0,4}, w Î {0,2}, x Î {0,1} % chmod 751 foobar.txt # specify all permissions – the following are different ways of setting "read-only" permission for a file % chmod =r file % chmod 444 file % chmod a-wx,a+r file University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 26 Various & Sundry UNIX file names are case-sensitive – assumption here: underlying file system is UNIX – e.g., myFile and myfile are two different names, and the logout command cannot be typed as Logout commands are available to change the owner and/or group of a file; e.g. chown, chgrp pager is a command (less, more) used to display a text file one page at a time % less unix.txt quickly create a file (or update the timestamp of an existing file) % touch unix.txt % ls –l uxix.txt -rw-r--r-- 1 zastre users 0 Aug 29 14:39 unix.tex University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 27 Introduction to UNIX (contd) The shell Basic command syntax Command types Getting help on commands I/O streams Redirection and pipelining Command sequences Console University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 28 the shell (again) The shell is the intermediary between you and the UNIX OS kernel It interprets your typed commands in order to do what you want – the shell reads the next input line – it interprets the line (expands arguments, substitutes aliases, etc.) – performs action There are two families of shells: – “sh” based shells, and “csh” based shells – they vary slightly in their syntax and functionality – we’ll use “bash”, the Bourne Again SHell (derivative of “sh”, known as the “Bourne shell”) – tip: you can find out what shell you are using by typing: echo $SHELL University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 29 basic command syntax % cmd [options] [arguments] – cmd represents here some builtin-shell or UNIX command – [options] = zero or many options – [arguments] = zero or many arguments option example opt a -opt -v --optname --verbose -opt arg -s 5 --optname arg --size 5 University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 30 basic command syntax (2) opt is a character in {a..zA..Z0..9} optname is an option name; e.g., --size, --keep argument, arg is one of the following: – file name – directory name – device name, e.g., /dev/hdb2 – number, e.g., 10, 010, 0x1af, … – string, e.g., "*.c", "Initial release", … – … University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 31 command types commands can be: – built into the shell (e.g., cd, alias, bg, set,…) – aliases created by the user or on behalf of the user (e.g., rm='rm –i', cp='cp –i', vi='vim') – an executable file binary (compiled from source code) script (system-parsed text file) Use the type command to determine if a command is builtin, an alias, or an executable. % type rm rm is aliased to ‘rm –I’ University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 32 some simple commands % cat [file1 file2...] – (catenate) copy the files to stdout, in order listed % less [filename] – browse through a file, one screenful at a time % date – displays current date and time % wc [filename] – (word count) counts the number of lines, words and characters in the input % clear University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 33 getting help on commands You can ask for help in several ways. Display a description of a shell command (builtin) % help times Display a long description of a command (from section n of manual) % man [n] chmod Display a one line description of a command ↳ % whatis gcc gcc gcc (1) - GNU project C and C++ compiler a mar Oral Use "info" % info gcc % info ls Many commands provide their own help % somecmd –h % somecmd --help University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 34 Write it down! University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 35  input & output streams each UNIX program has access to three I/O “streams” when it runs: – standard input or stdin; defaults to the console keyboard – standard output or stdout; defaults to the console screen – standard error or stderr; defaults to the console screen the shell provides a mechanism for overriding this default behaviour (stream redirection) University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 36 stream redirection redirection allows you to: – take input from a file – save command output to a file redirecting from/to files using bash shell: – stdin: % cmd < file % less < ls.1 – stdout: % cmd > file # write % ls -la >dir.listing % cmd >> file # append % ls –la /home >>dir.listing – stderr: % cmd 2> file # write % cmd 2>> file # append University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 37 stream redirection (2) redirecting stdin and stdout simultaneously % cmd < infile > outfile % sort < unsorted.data > sorted.data redirecting stdout and stderr simultaneously % cmd >& file % grep 'hello' program.c >& hello.txt % cmd 1>out.log 2>err.log UNIX gotchas: – symbols used for redirection depend on shell you are using – our work will be with the Bash shell (bash, sh) – slight differences from C-shell’s (csh, tcsh) University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 38 pipes Pipes are considered by many to be one of the major Unix-shell innovations – excellent tool for creating powerful commands from simpler components, – does so in an effective, efficient way. Pipes route standard output of one command into the standard input of another command Allows us to build complex commands using a set of simple commands Motivation: – without pipes, lots of temporary files result University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 39 without pipes Example: How many different users are currently running processes on the server? University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 40 without pipes Example: How many different users are currently running processes on the server? % ps aux > temp1.txt % awk '{ print $1 }' temp1.txt > temp2.txt % sort temp2.txt > temp3.txt % uniq temp3.txt > temp4.txt % wc –l < temp4.txt > temp5.txt % cat temp5.txt Off by one – need to mentally subtract one from the resulting number University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 41 with pipes Example: How many different users are currently running processes on the server? % ps aux | awk '{ print $1 }' | sort | uniq | wc –l % ps aux | awk '{ print $1 }' | sort | uniq | wc –l | xargs expr -1 + Note the structure of the command: – “generator” command is at the head – successive “filter” commands transform the results – this is a very popular style of Unix usage University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 42 A bit more about pipes Pipes can save time by eliminating the need for intermediate files Pipes can be arbitrarily long and complex All commands are executed concurrently If any processing error occurs in a pipeline, the whole pipeline fails University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 43 command sequencing multiple commands can be executed sequentially, that is: cmd1;cmd2;cmd3;…;cmdn % date; who; pwd may group sequenced commands together and redirect output % (date; who; pwd) > logfile note that the last line does not have the same effect as: % date; who; pwd > logfile University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 44 Introduction to UNIX (contd) Filename expansion Command aliases Quoting and backslash escapes bash command history Job control Shell/environment variables Customizing your shell University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 45 filename expansion "shorthand" for referencing multiple existing files on a command line * any number of characters ? exactly one of any character [abc] any character in the set [abc] [!abc] any character not in the set [abc] these can be combined together as seen on the next slide University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 46 filename expansion (2) examples: – count lines in all files having the suffix ".c" % wc -l *.c – list detailed information about all files with a single character file extension % ls -l *.? – send to the printer all files with names beginning in Chap* and chap* files % lpr [Cc]hap* University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 47 filename expansion (3) * matches any sequence of characters (except those with an initial period) % rm *.o # remove all files ending in '.o' % rm * # remove all files in directory % rm../*-old*.c ? matches any single character (except an initial period) % rm test.? # remove test.c and test.o (etc.) So to delete a file of the form ".filename" you can’t use wildcards % rm.viminfo How do we delete a file named *? University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 48 quoting controls bash’s interpretation of certain characters what if you wanted to pass '>' as an argument to a command? strong quotes – All characters inside a pair of single quotes (') are preserved. weak quotes – Some characters ($,`) inside a pair of double quotes (") are expanded (interpreted) by the shell. backquotes – substitute result of evaluation as a command University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 49 quoting % echo $SHELL * /bin/bash file1 file2 file3 % echo '$SHELL' '*' $SHELL * % echo "$SHELL" "*" /bin/bash * %echo `date` Fri May 12 11:54:27 PDT 2017 University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 50 backslash escaping Characters used by bash which may need to be escaped: ~, `, #, $, &, *, (, ), \, [, ], {, }, ;, ', ", , /, ?, ! single characters can be "protected" from expansion by prefixing with a backslash ("\") cmd \* is the same as typing cmd '*' protecting special characters in such a manner is an example of backslash escaping % cp ~bob/junk \* # make copy of junk named '*' % rm '*' # remove '*' (not "delete all files") Single quotes around a string turn off the special meanings of most characters % rm 'dead letter' % cp University ~bob/junk '*' # same as up SENG265: of Victoria above Software Development Methods Department of Computer Science Introduction to UNIX: Slide 51 command substitution backquotes (`) are used to substitute the result of evaluating a command into a quoted string or shell variable: % echo "Current date is: `date` " Current date is: Fri May 12 11:54:27 PDT 2017 % BOOTTIME=`date` % echo $BOOTTIME Fri May 12 11:54:27 PDT 2017 standards-compliant (i.e. POSIX) style avoids backticks: % echo "Current date is: $(date)" Current date is: Thu 15 Sep 2011 14:58:13 PDT University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 52 bash command history bash (and other shells like sh, tcsh, ksh, csh) maintain a history of executed commands uses the readline editing interface history will show list of recent commands % history # print your entire history % history n # print most recent n commands % history –c # delete your history a common default size of the history is 500 commands – and the history is usually remembered across login sessions University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 53 Using history simple way: use up and down arrows using the “!” history expansion % !! repeat last command % !n repeat command number n % !-n repeat the command typed n commands ago % !foo last command that started with foo University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 54 shell variables a running shell carries with it a dictionary of variables with values some are built in and some are user defined used to customize the shell use env to display the values of your environment variables use set to display the values of your environment + shell variables % env PWD=/home/bgates GS_FONTPATH=/usr/local/fonts/type1 XAUTHORITY=/home/dtrump/.Xauthority TERM=ansi HOSTNAME=c70... University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 55 shell variables (2) many variables are automatically assigned values at login time variables may be re-assigned values at the shell prompt new variables may be added, and variables can be discarded assigning or creating a variable – and notice absence of spaces around the "=" symbol: % somevar="value" to delete a variable: % unset somevar To use the value of a shell variable use the $ prefix: % echo $somevar University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 56 PATH shell variable helps the shell find the commands you want to execute its value is a list of directories separated by ':' symbol when we intend to run a program, the directory of its executable should be in the PATH in order to be found quickly Example: assume that program cmd is located in directory "/usr2/bin“ % echo $PATH PATH=/usr/bin:/usr/sbin:/etc % cmd bash: cmd: command not found % PATH="$PATH:/usr2/bin" % echo $PATH PATH=/usr/bin:/usr/sbin:/etc:/usr2/bin % cmd (... now runs...) the shell searches sequentially in the order directories are listed University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 57 environment variables some shell variables are exported to every subshell – when executing a command, the shell often launches another instance of the shell; this is called a subshell % (date ; who ; pwd) > logfile – the subshell executes as an entirely different process – the subshell “inherits” the environment variables of its “parent” (main shell) “exporting” shell variables (var) to the environment % export var % export var=value example: % export EDITOR=vim University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 58 customizing the shell In your accounts there will be two files you can modify to customize the bash shell “~/.bash_profile” is evaluated by the shell each time you login to your account. by default, “~/.bash_profile” sources (reads and evaluates) a second file “~/.bashrc” conventional wisdom suggests that permanent shell/environment variables should be placed in “~/.bash_profile”, and aliases should be placed in “~/.bashrc” system administrators, for very sound reasons, often prefer that we don’t modify “~/.bash_profile”, but instead customize the shell by modifying “~/.bashrc” (adding shell variables, aliasing, etc.) In both cases, the changes you make to these files will not take effect until you source the modified file % source.bashrc Use with caution! University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 59 job control the shell allows you to execute multiple programs in parallel starting a program in the background... % cmd & 3141 # (jobid=1,pid=3141)... and bringing it to the foreground % fg %1 placing a running program in the background % cmd ^Z % bg %1 University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 60 job control (2) stopping and restarting a program: % vim hugeprog.c ^Z + Stopped % jobs + Stopped vim hugeprog.c % gcc hugeprog.c –o hugeprog & 2435 % jobs - Stopped vim hugeprog.c + Stopped gcc hugeprog.c –o hugeprog % fg %1 vim hugeprog.c terminating (or “killing”) a job: % kill %n # use kill -9 %n if the job won’t die! % kill %cc # kill job that starts with cc University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 61 job control (3) job states University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 62 shell scripting Why write a shell script? – Sometimes it makes sense to wrap a repeated task into a command. – Sequences of operations can be placed in a script and executed like a command. Not everything is sensible for a script, though – For some problems it would make more sense for a full program – Instances: resource-intensive tasks, complex applications, mission critical apps, etc. University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 63 Some simple scripts For what appears below, ensure the file containing the script is executable #!/bin/bash hello.sh echo 'Hello, world!' #!/bin/bash status.sh uptime users The very first line is called a "shebang" path – What follows the "#!" ("shebang") is the command that interprets everything else that follows. University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 64 Echoing command-line arguments Obtaining command-line argument is relatively straightforward #!/bin/bash status.sh echo "First command-line arg" $1 echo "Second command-line arg" $2 Notice that echo takes multiple strings – But these are not separated by commas – (Shell syntax is close enough to regular programming syntax to be confusing.) University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 65 Selection The numeric representation of true and false are inverted – True == 0 – False == anything else Common tests involve file operators Note the spacing in the test expression! #!/bin/bash if [ -e /home/zastre/seng265/assign1 ] ; then echo 'Hooray! The file exists!' else echo 'Boo! The file ain't yet there…' fi # Other tests: # -f True if file is a regular file # -d True if file is a directory # -w True if file exists and is writable # -O True if the account running script owns the file University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 66 String and arithmetic relationals String operators compare lexical order (i.e., dictionary order) Arithmetic operators only work on integers Note spacing variants… #!/bin/bash if [ "abc" != "ABC" ] ; then echo 'Case does matter here' ; fi if [ 12 \< 2 ] ; then echo 'Why is 12 less than 2??!' ; fi a=12 b=2 if [ "$a" -lt "$b" ] then echo 'Something is wrong with numbers here...' else echo University "Ah ha! $a can be either a string or integer" SENG265: Software Development Methods of Victoria fi Department of Computer Science Introduction to UNIX: Slide 67 More generate tests Sometimes we could use a good old logical OR or logical AND Some C-like expressive power is possible (Note slight syntax variation with the "if" statement) #!/bin/bash if [[ -e ~/.bashrc && ! -d ~/.bashrc ]] then echo 'Definitely a config file to process' fi University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 68 Arithmetic? Not so good… Bash usually treats variables as strings We can force bash to treat a variable's value as an integer #!/bin/bash x=1 x=$x+1 echo $x # Still a string y=1 (( y=y+1 )) echo $y # This gets it... University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 69 Iteration List-like values are common in the shell – Arguments passed to command – Files expanded by wildcards A list literal is simply spelled out #!/bin/bash for x in 1 2 a do echo $x done for x in * do echo $x done University of Victoria SENG265: Software Development Methods Department of Computer Science Introduction to UNIX: Slide 70 Iteration The output from commands may be used to create a list – Note that the text from the command will be tokenized into a sequence of individual words… We can also write old-style for loops Use "set –x" to have the shell print out commands as they are executed – Handy for debugging #!/bin/bash set –x for i in $(date) ; do echo item: $i done for (( i=0; i